The use of thread-forming screws for attachment of a component to a molded component offers both material and production cost advantages over free running fasteners since a free running fastener requires a metallic insert. By avoiding the need for an insert, the use of thread-forming screws for component attachment provides additional advantages due to cycle time reduction and lower tooling costs.
The mechanism that allows the forming of the threads around the fastener is the local melting and reforming of the female threads around the male thread. Due to creep of the plastic with exposure to temperature, there is intrinsic relaxation of the material, resulting in reduced bolt stretch and consequential loss of clamp load at the joint.
An example of an assembly requiring fasteners is where certain components are attached to the intake manifold of an internal combustion engine. Components commonly associated with the intake manifold include the throttle body, the mass air flow sensor, various ducts and a fuel rail. Some of these components, such as the automotive throttle body have a flange for coupling with a flange located on an inlet side of the intake manifold. It is very common for today's throttle body to be composed of a metal such as aluminum, while it is very common for the modern intake manifold to be injection molded of a polymerized material.
Efforts have been made in the past to use thread-forming screws to attach one of these components, such as the throttle body, to the molded intake manifold. The use of thread-forming screws to attach the throttle body to the manifold presents certain challenges as this clamp load is lost, since it utilizes a face seal. In order to mitigate abrasion of the seal due to relative motion between the intake and the throttle body faces, shear pins are often used.
A further challenge is created when two parts are to be connected despite manufacturing tolerances. Typically the fastener-passing bores of the flange being fastened are slightly oversized. As a result, when, for example, a throttle body is attached to an intake manifold, the inside diameters of the throttle body and the intake manifold may be slightly offset because the fastener is loose relative to the oversized fastener-passing bore. As a result, as air travels from the throttle body into intake manifold, the mismatch may lead to eddies being formed and may lead to a whistling sound that is noticeable to the operator of the vehicle. Such a mismatch also slightly negatively impacts the maximum amount of flow to the engine, i.e., slightly hurting peak engine performance.
Thus, while presenting a partial solution to the challenges faced in the use of thread-forming screws, the shear pins must be sized and their tolerances adjusted such that assembly resulting in a proper alignment is always possible. However, this approach limits the effectiveness of the use of shear pins in preventing relative motion. Some designs exist that utilize split pins or crush features on the edge of the pins to allow a tighter fit into the fastener-passing bore. However, this approach raises the concern of how much material can be deformed or moved without causing interference in the joint seating or in the gasket sealing function.
Thus known approaches to attaching a component to a molded part are undesirable and impractical. Accordingly, an improved arrangement for attaching a component to a molded part remains wanting.